Does the factory reflash CPU for gears?
Thread Starter
JK Newbie
Joined: Mar 2008
Posts: 77
Likes: 0
From: Whittier, California
Does the factory reflash CPU for gears?
Looks to me like the Jk is available with 3.21, 3.73, and 4.10 gears depending on model and options. Does the factory flash the CPU at the time of the build for the specific gears in each JK? Is there a big enough difference between 3.21 and 4.10 to throw the CPU off if it is not flashed and what would this do to the computer and the speedomter?
Here is why I ask.
In Kalifornia when we do a smog check it is done on rollers. If the vehicle does not hit the specified rpm at 15 mph and 25 mph it will not pass. I would suspect that all of the different combinations between 6spd and auto and 3.21, 3.73, 4.10 would make for at least six different rpms for those speeds from the factory. I intend on lifting my JK and installing 35" tires and regearing to at least 4.88 but probably 5.13 so I am trying to figure out if I would be better off starting with the base 3.21 or upgrade to the 3.73 or 4.10.
In the end I still need it to pass smog. I would prefer to start with the basic 3.21 gears since I want to change them anyway but I am thinking I may have to pop for at least the 3.73 to get the factory cruising rpm high enough to be in the right range to still pass smog with lift, tires and gears. If this does not make sense let me know and I will try to re-ask.
Thanks
Here is why I ask.
In Kalifornia when we do a smog check it is done on rollers. If the vehicle does not hit the specified rpm at 15 mph and 25 mph it will not pass. I would suspect that all of the different combinations between 6spd and auto and 3.21, 3.73, 4.10 would make for at least six different rpms for those speeds from the factory. I intend on lifting my JK and installing 35" tires and regearing to at least 4.88 but probably 5.13 so I am trying to figure out if I would be better off starting with the base 3.21 or upgrade to the 3.73 or 4.10.
In the end I still need it to pass smog. I would prefer to start with the basic 3.21 gears since I want to change them anyway but I am thinking I may have to pop for at least the 3.73 to get the factory cruising rpm high enough to be in the right range to still pass smog with lift, tires and gears. If this does not make sense let me know and I will try to re-ask.
Thanks
JK Enthusiast
Joined: Dec 2007
Posts: 478
Likes: 0
From: North East TX
The hypertech programmer allows you to adjust for gear ratio as well as tire size. I'm sure the CPU setting has to be set for each option at the factory before leaving. If they didn't set to a specific gear ratio the speedo would be off.
JK Enthusiast
Joined: Feb 2007
Posts: 234
Likes: 0
From: Yorkville, IL
The JK uses independent wheel sensors to determine how fast the Jeep is going, so it doesnt care how fast the engine or gears are spinning to make it go a certain speed.
Changing the tire size is the only thing that effects the speedo, or a programmer to correct the tire size in the PCM settings.